Generally, a nonaqueous secondary battery, such as a lithium ion secondary battery, is composed of a positive electrode obtained by forming, on a current collector, an active material layer containing a positive electrode active material such as lithium cobalt oxide or a like lithium compound, a negative electrode obtained by forming, on a current collector, an active material layer containing a negative electrode active material such as graphite or a like carbon material capable of occluding/releasing lithium, a nonaqueous electrolytic solution obtained by dissolving an electrolyte such as LiPF6 or a like lithium salt in an aprotic nonaqueous solvent, and a separator formed of a porous polymer membrane.
Nonaqueous secondary batteries have high energy densities. Because of this characteristic, they are widely spread as main power supplies for portable electronic devices, such as mobile phones and laptop computers. Further, in recent years, with the increasing global interest in the problem of global warming, they have come to be used for electric cars, hybrid cars, and the like for the purpose of reducing CO2 emissions.
In ensuring the safety of a nonaqueous secondary battery, a separator plays an important role. Especially in terms of the shutdown function, a polyolefin microporous membrane containing a polyolefin as a main component is currently used. Incidentally, a shutdown function refers to the following function: when a battery is overheated, the polyolefin melts and closes pores in the microporous membrane to block the migration of lithium ions, thereby preventing further heat generation.
However, when a separator formed only of a polyolefin microporous membrane is exposed to a temperature higher than the temperature at which the shutdown function is developed, the entire separator may melt (so-called meltdown) to cause an internal short-circuit, thereby inducing rapid heat generation or explosion of the nonaqueous secondary battery.
In addition, a polyolefin resin has poor adhesion with other materials such as electrode materials. As a result, due to the insufficient adhesion between a polyolefin microporous membrane and electrodes, a gap may be formed between the electrodes and the separator, thereby causing a decrease in battery capacity or the degradation of cycle characteristics.
Thus, for the purpose of improving the adhesion between electrodes and a separator, it has been proposed to provide a porous layer containing an adhesive resin on one side or both sides of a polyolefin microporous membrane. In particular, as a technique for enhancing the adhesion between electrodes and a separator, a separator obtained by forming a porous layer containing a polyvinylidene fluoride resin as a main component on a polyolefin microporous membrane is known (e.g., PTL 1).